1. Field of the Invention
The present invention relates to a sheet processing apparatus having a punching unit that punches a hole in a sheet outputted from an image forming apparatus such as a copier or a laser beam printer.
2. Description of the Related Art
A conventional sheet processing apparatus punches a hole in a recording sheet, on which an image is formed, one by one during conveyance (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2007-76775 (JP 2007-76775A)).
FIG. 10 is a sectional view of the above-mentioned conventional sheet processing apparatus. As shown in FIG. 10, the conventional sheet processing apparatus 1a is connected to a sheet ejection side of an image forming apparatus 300 that forms an image on a sheet. The sheet processing apparatus 1a is provided with a punching device 50 that punches a hole in a sheet on which an image is formed by the image forming apparatus 300 and that is conveyed.
FIG. 11 is a schematic view showing the punching device 50 in FIG. 10 when it is viewed from the right side, i.e., the side of the image forming apparatus 300. As shown in FIG. 11, the punching device 50 is provided with an entrance sensor 31, a punching unit 51, and a shift unit 42 that are arranged in a conveying direction of a sheet. Further, the punching device 50 is provided with a conveying motor M1, a sensor-moving motor M3, and a shift-roller-moving motor M2.
The conveying motor M1 rotates a shift roller pair 4 through a broad gear 40, and rotates a shift roller pair 3 connected by a timing belt 41. These shift roller pairs 3 and 4 are mounted on the shift unit 42. The shift unit 42 is shifted in directions of arrows A and B by the shift-roller-moving motor M2.
A position of a sheet under conveyance in a width direction is detected by a lateral registration detection sensor 32 arranged between the shift roller pair 3 and the shift roller pair 4. The lateral registration detection sensor 32 is mounted on a sensor unit 33. The sensor unit 33 is moved in directions of arrows 43 and 44 by the sensor-moving motor M3.
FIG. 12 is a timing chart showing a punching operation by the punching device 50. Time advances to right from left in the figure.
FIG. 12 shows driving signals of the conveying motor M1, the sensor-moving motor M3, the shift-roller-moving motor M2, and the punching motor (not shown), respectively. In each driving signal, a high (H) level means that a corresponding motor is under operation, and a low (L) level means that it is not under operation.
First, the sensor-moving motor M3 is operated so that the sensor 32 moves in a direction (referred to as a “width direction”, hereinafter) that crosses the conveying direction of a sheet (an operating section 70) at a timing when the lateral registration detection sensor 32 detects a front edge of the sheet conveyed from the image forming apparatus 300. A position of the sheet in the width direction is detected based on a moving distance of the sensor 32 between the start of moving and the detection of the sheet. Next, the shift-roller-moving motor M2 is operated to move the sheet to a predetermined position in the width direction so as to punch a hole at an aimed position in the sheet (an operating section 66). And when a rear edge of the sheet reaches a punching position, the sheet is stopped, and the punching motor is operated to punch a hole in the sheet (an operating section 71). When stacking the punched sheets on the stack tray 201, stacked positions of respective groups of the sheets are changed in order to clarify boundaries among the groups (an operating section 65). Thus, the change of the stacked positions to clarify the boundaries among the groups is hereafter called a “sheet ejection offset”.
When the rear edge of the sheet passes the shift roller pair 4, the shift-roller-moving motor M2 is operated so that the shift roller pairs 3 and 4 are moved to standby positions for the next sheet (an operating section 72). The operations described above are a series of punching operations.
It should be noted that the front edge of the sheet reaches the shift roller pair 3 at a timing 68 in FIG. 12. The shift roller pairs 3 and 4 return to the standby positions at a timing 69. Therefore, a punching process time is defined as a period (a section 67) from the time when the front edge of the sheet reaches the shift roller pair 3 to the time when the shift roller pairs 3 and 4 return to the standby positions after the rear edge of the sheet passes the shift roller pair 4.
Thus, the above-mentioned conventional sheet processing apparatus achieves downsizing and cost reduction of the device by arranging the punching unit 51 near the shift roller pair 3 and by using the same mechanism for moving the sheet to adjust the sheet to the punching position and for moving the sheet for the sheet ejection offset.
Incidentally, the sheet processing apparatus including the punching unit is required to increase the speed of the punching process in order to create a lot of products in a short time. The shorter the punching process time is, the higher the speed of the punching process is. In order to shorten the punching process time, it is effective to return the shift roller pairs to the standby positions quickly after punching. In order to return the shift roller pairs to the standby positions quickly, it is necessary to shorten a period until the rear edge of the sheet passes the shift roller pair after finishing the punching (referred to as a “shift conveying time”, hereinafter). In order to shorten the shift conveying time, a method to increase the sheet conveying speed of the sheet after punching, a method to reduce the number of shift roller pairs from two pairs to one pair, etc. can be considered, for example.
When either of the above-described methods is executed, the timing when the rear edge of the sheet passes the shift roller pair 4 is changed to a position 74 from a position 73 in the timing chart in FIG. 12. As a result, in the above-mentioned conventional sheet processing apparatus, the rear edge of the sheet passes the shift roller pair 4 before the sheet ejection offset (the operating section 65 in FIG. 12) is completed. This causes a problem that the movement of the sheet by the shift roller pairs 3 and 4 in the width direction for the sheet ejection offset becomes insufficient.